The objective is to find out what the physiological controls of water drinking are under spontaneous, unstressed conditions when the animal has both water and food freely available. Current concepts of thirst are largely based on experiments on water deprived or depleted animals, and there is doubt as to how such controls operate under spontaneous conditions. Spontaneous drinking is associated with eating, but the link between eating and drinking is poorly understood. Specific aims are: 1. To measure central venous and right atrial pressures during spontaneous eating and drinking in undisturbed pigs, free to move about, eating and drinking operantly, using miniature pressure transducers. 2. To evaluate the role of the renin-angiotensin system (RAS) in spontaneous drinking by studying the effects of infusions of angiotensin II, of captopril, RAS activation by other stimuli such as isoproterenol, and measurement of RAS components in plasma. 3. Evaluation of sympathetic nervous system (SNS) activity, which can cause release of renin, in terms of heart rate, arterial blood pressure, and catecholamine concentrations. 4. Dry mouth, a possible subtle cue for drinking, will be assessed using impedance measurements within the oral cavity and correlated with drinking. 5. Duodenal osmolality might act either as a stimulus to drinking (hyperosmotic conditions) or an inhibition (hypoosmotic conditions). Duodenal samples can be obtained via implanted catheters and duodenal osmolality can be varied with hypertonic or hypotonic infusions. 6. To study blood volume changes associated with eating and drinking and the role of the spleen in adjusting total blood volume by releasing, or taking up , red cells. 7. To determine the mechanism of insulin-induced drinking and its possible role in meal-associated drinking. A better understanding of the physiology of meal-associated drinking will be useful-in alleviating depressed food intake in infants, during recovery from disease, in the aged, and whenever food or water intake is disturbed. The pig is an excellent model for the human. Knowledge of such a fundamental process as ordinary water drinking surely will have wide applications in medical management of conditions where water or food intake are disturbed.